Known friction clutches provide a releasable torsional connection between a motor vehicle engine flywheel and an associated transmission. Repeated clutch disengagement and engagement cycles wear the friction material of the clutch driven disc. The wear results in a change in the axial location of the pressure plate in the engagement position. The shift in axial location results in a decrease in the clutch engagement force or clamping load. Clutches are commonly provided with adjustment mechanisms to compensate for such wear.
Clutches achieve the required clamping loads by applying a spring load to the pressure plate. One type of clutch applies a spring load to a plurality of radially oriented levers which in turn engage the pressure plate. If compressive coil or angle springs are employed, the spring load is commonly applied to radially inner ends of the levers. The levers are pivotally supported at the radially outer ends. As the friction material wears, the engaged position of the radially inner lever tips moves closer to the flywheel. Adjustment mechanisms disposed between the cover and the levers compensate for this change by increasing the axial distance of the levers' pivot points from the cover. Alternatively, the change in friction material thickness is compensated for by providing an adjustment mechanism on the pressure plate instead of the cover. Clutches may employ diaphragm springs in place of coil springs. Some types of diaphragm springs may have radially inwardly directed fingers in place of multiple piece lever and spring arrangements. The fingers act as the levers of the heavy duty clutches in that they are axially displaced at their radially inner most tip to release the clutch.
Known adjustment mechanisms typically rely on the relative rotation of two parts having inclined surfaces in common engagement with each other. The relative rotation of these elements compensates for wear of the friction material. One strategy for compensating is to axially displace the pivot ends of the levers away from the cover and closer to the flywheel, thereby reducing the amount of spring deflection required to achieve a full clutch engagement. Another strategy is to axially displace the pressure plate's point of contact with the levers away from an engagement surface of the pressure plate, thereby compensating for friction material wear. The ease of adjustment is greatly affected by the coefficient of friction between the relatively rotating parts. When the engaged surfaces become contaminated by debris from the clutch, it increases the difficulty of properly adjusting the clutch to compensate for any wear of the friction elements. The configuration of the clutch allows, and indeed promotes the circulation of debris through the clutch. Spinning the clutch cenfrigually forces or impels air and airborne debris through large openings in the clutch cover. Debris may include facing dust, cast iron dust, fretting material and external road dirt, moisture and/or salt.
It is desired to provide a clutch with an adjusting mechanism which is relatively insensitive to the debris commonly found in clutches.
It is also desired to provide a clutch with a shield protecting the adjustment mechanism from the entry of debris there into.